FIG. 13 schematically illustrates a multi-carrier transmission system of this type. A serial digital data stream a is supplied to a serial/parallel converter 1, which divides the serial digital data stream a into data packets comprising N/2 sub-packets, N being an integer. Each data packet is transmitted in parallel to an encoder 2, which modulates each of the sub-packets to an individual carrier having a predetermined carrier frequency (“tone”), using, for example, the quadrature amplitude modulation (QAM) method. A first digital signal vector, which is supplied to a device 3 for carrying out an inverse Fourier transformation, is thereby generated. A second digital signal vector, which comprises N sampled values of a transmission signal, is generated by means of the inverse Fourier transformation of the first digital signal vector. A prefix and a suffix are optionally (for example, in the case of VDSL) also added to this second digital signal vector, and the second digital signal vector is then supplied to a parallel/serial converter 4, which issues the corresponding sampled values in series. The second digital signal vector is also referred to as an IFFT frame or a DMT frame.
The sampled values are filtered using a digital filter 35, converted into an analog signal using a digital/analog converter 36 and amplified in a line driver 37. The resulting analog transmission signal is transmitted via a transmission channel 38. During transmission through the transmission channel 38, a noise b is added to the signal, symbolised by an adder 39. On the receiver side, the signal thus received is supplied to an assembly 40, which comprises an equalizer, a filter and an analog/digital converter. The signal is then decoded in that substantially the opposite steps to those on the transmission side are carried out by means of the blocks 1 to 4, for which purpose a serial/parallel converter 30, a device 31 for carrying out a Fourier transformation, a decoder 32, a slicer or decision element 33 and a parallel/serial converter 34 are provided. Finally, the parallel/serial converter 34 issues a received data stream a′, which, provided that no transmission errors occur, corresponds to the transmission data stream a.
A communication system of this type is known, for example, from U.S. Pat. No. 6,529,925 B1.
As the transmission signal transmitted via the transmission channel 38 is composed of a large number of different signals, in particular sinusoidal signals, having various carrier frequencies, the respective amplitudes and phases of which are determined by the serial data stream a and therefore do not exhibit any predetermined relationships to one another, the amplitude of the transmission signal exhibits approximately a Gaussian distribution. Curve 41 from FIG. 14 displays the probability p of the occurrence of an amplitude A of the transmission signal, which probability was calculated by means of a simulation of a transmission signal having a frame length of 256 and modulated using the discrete multitone modulation (DMT) method.
Within this Gaussian distribution, the crest factor of the transmission signal is relatively high, i.e. very high maximum amplitudes, compared with the average amplitude value, may occur. As the blocks 35, 36, 37 and 39 from FIG. 13, in particular the digital/analog converters or analog/digital converters and the line drivers, have to be configured for processing all of the possible amplitude values, i.e. also the maximum amplitude values, relatively complex implementation, which requires a large chip area and thus entails additional costs, is in this case necessary. It is therefore desirable to reduce the crest factor, in particular the maximum amplitude.
In addition to methods that reduce the maximum amplitude at the cost of a disturbance of the signal, methods that use one or more of the carrier frequencies in order to modify the transmission signal such that the maximum amplitude is reduced are also known in this regard. The carrier frequencies used for this purpose may not be used, or may be used only partially, for the actual data transmission.
U.S. Pat. No. 6,424,681 B1, for example, discloses a method for reducing the crest factor using a plurality of carrier frequencies. These carrier frequencies are preferably distributed uniformly over the total usable frequency range. A standardized correction signal, known as a kernel, which exhibits as “Dirac-like” a form as possible, i.e. substantially comprises a single maximum, is generated from these carrier frequencies. In order to correct a transmission signal, the phase and the amplitude of this correction signal are adapted using a suitable scaling factor. The correction signal thus adapted is deducted from the transmission signal, in a manner that may be iteratively repeated in order to reduce a plurality of peak values of the transmission signal.
U.S. Pat. No. 6,529,925 B1 discloses a method for reducing a crest factor in which only the Nyquist frequency, i.e. the final frequency of the inverse Fourier transformation, which in ADSL systems is not used for data transmission, is used as a carrier frequency for the correction. However, as only a single carrier frequency is used for the correction, the capacity of this method is limited. This method may also not be applied to VDSL signals, as the Nyquist frequency is, in this case, outside the usable frequency range, both for downstream and for upstream transmission.
In the case of VDSL systems, in particular, there is the further problem that, at 8,192 sampled values, the frame length, i.e. the number of sampled values in a DMT frame, is very large, and a correction signal or a correction vector comprising 8,192 sampled values must accordingly also be calculated, and this is relatively complex.